High-Strength Steel in Structural Concrete · A615 and A706 A615 and A1035 A1035 Grade 100 High Strength. Key Parameters Yield strength Tensile strength Tensile-to-yield strength

High-Strength Steelin Structural Concrete:

From Research to Building Codes

Project Sponsors

• American Concrete Institute (ACI)ACI Foundation – Concrete Research Council

• Charles Pankow Foundation• Concrete Reinforcing Steel Institute (CRSI)

Commercial Metals CompanyMMFX Technologies CorporationNucor Corporation

• Electric Power Research Institute (EPRI)

Project Participants

• Collaborators• David Darwin• Matt O’Reilly

• Graduate Students• Ali Ajaam• Abdal Al-Sabawy• Shahedreen Ameen• Krishna Ghimire• Eduardo Guillen

• Muna Hano• Sajed Huq• Yun Shao• Jayne Sperry• Don Spradling

• Alex Weber-Kamin• Samir Yasso

• Many others…

Overview

• High-Strength Steel (HSS) as Reinforcement• HSS in slender structural walls• HSS in coupling beams• HSS as shear reinforcement• Bond and development of HSS• Summary

Overview


Motivation

Authors’ Collection

Motivation

Photo courtesy of E. A. Burgos

Motivation

Authors’ Collection

Motivation

Photo courtesy of K. K. Mow

Motivation

Authors’collection

Potential Benefits of High-Strength Steel

• Cost savings• Reduced material quantities• Reduced reinforcement congestion• Improved quality of construction

High-Strength Steel

Usually defined as having a yield strength fy of 80 ksi or greater.

Code Limitations

0

20

40

60

80

100

1956 1963 1971

Desig

n f y

,ksi

ACI 318 Version

Shear/Torsion

Flexure/Axial

0

20

40

60

80

100

1956 1963 1971 2008

Desig

n f y

,ksi

ACI 318 Version

Shear/Torsion

Flexure/Axial

Confinement

Seismic

High Strength

Background

ACI ITG-6, 2010 ATC 98, 2014 ATC 115, 2014

0

40

80

120

160

200

0 0.02 0.04 0.06 0.08 0.1 0.12 0.14 0.16 0.18

Stre

ss (k

si)

Strain

Conventional vs. High-Strength Steels

Grade 60

Grade 120

Grade 80

Grade 100

0.2% offset

TY

su Uniform

A615 and A706

A615 and A706A615 and A1035

A1035

ddddddGGGGGGGGGGGGGGrrrrrrraaaaaaaaddddddddddeeeeeeee 111111111000000000000000000000GGGGGGGGGGGGGrrrrrrrraaaaaaaaadddddddddeeeeeeee 1111111100000000000000000000GGGGGGGGrrrraaaaaddddddeeeee 11111100000000000High Strength

Key Parameters

Yield strength

Tensile strength

Tensile-to-yield strength ratio

Uniform elongation

Fracture elongation su

sf

fy

ft

T/Y

Main Concerns in DesignGravity Design• Crack control• Deflection control• Development length• Moment redistribution• Shear strength

Seismic Design• Strength, stiffness, and

deformation capacity• Spacing of hoops to

delay bar buckling• Bond stresses

KU selected for testsof walls and coupling beams

ATC 115

Overview


Objectives

(1) Evaluate the effect of tensile-to-yield strength ratio (T/Y) of steel bars on wall deformation capacity

(2) Determine the minimum uniform elongation ( ) required of bars used in earthquake-resistant construction

T-shaped Wall Specimens

Id. fy (ksi) T / Y f’c (ksi)T1 60 1.25

8T2 100 1.1T3 100 1.2 1.3T4 100 1.3

Similar fyhw/ w = 3

P

Tensile-to-Yield Strength Ratio (T/Y)

0

20

40

60

80

100

120

140

0.00 0.01 0.02 0.03 0.04 0.05 0.06

Stre

ss,k

si

Strain

T/Y = 1.1 A706 Grade 60 T/Y = 1.4 0.2% Offset

fy = 100 ksi su < 10%

fy = 60 ksi su > 10%

T/Y = 1.25

Cross Section

T1 T2, T3, T4*

Stem Boundary Element

T1 T2, T3, T4

T-Shaped Wall Specimens8'-4"

34"

CLR

, TY

P

#4 @ 15", HORIZ

5'-0

"2'

-6"

10"

1'-3"

10"

1'-3"

(14) #6, VERTS

#4 @ 15", HORIZ

(14) #4, VERTS

(6) #6, VERTS

Neutral axis

Construction

Test Setup

T3

T1

Drift Ratio

= ×

Loading Protocol

-6

-4

-2

0

2

4

6

Drif

t Ra

tio, %

1 Step

1 Cycle

Stepa Drift, %1 0.20

2 0.30

3 0.50

4 0.75

5 1.00

6 1.50

7 2.00

8 3.00

9 4.00a Two cycles per step

Shear vs. Drift Ratio

T1 – 60 ksi T3 – 100 ksi

18 -12 -6 0 6 12 18

-6 -4 -2 0 2 4 6

Top Displacement, in.

Drift Ratio, %

T

C

T

C

-18 -12 -6 0 6 12 18

-320

-160

0

160

320

-6 -4 -2 0 2 4 6

Top Displacement, in.

Shea

r, k

Drift Ratio, %

T

T

C

C

Bar fracture

Mn

Test Results

Bar buckling occurred during the 2% drift cycle in both T1 and T3

T3 – 100 ksi

Test Results

T1 – 60 ksi T3 – 100 ksiBar buckling

Test Results

T3 – 100 ksiT1 – 60 ksi4% Drift Ratio

Barfracture

Test Results

Test Results

T2 (Grade 100, su = 5.5%, T/Y = 1.10)2% Drift Ratio

T2 – 100 ksi

#4#6

Mn

Lug base radiusr < 0.25 h

Fractured No. 4 Bar from T2

Special Attributes of No. 4 Bars in T2(1) Sharp edges in deformation pattern of longitudinal bars.

(Reported poor performance in low-cycle fatigue tests)(2) Lowest uniform elongation su.

(Measured su was 5.5%)(3) Lowest T/Y ratio.

(Measured T/Y was 1.10)(4) Strain gauges on No. 4 bars at the wall-base interface.

(Potentially creating a weak plane)(5) Zero percent Vanadium.

(Affecting low-cycle fatigue and strain aging)

Wall Tests

0 2 4 6 8 10 12 14 16

Uniform elongation, %

0

1

2

3

4

5

6D

rift

ratio

, %

T1

T2

T3

T4

T5

T6

Wall T/Y 1/0.85 (or 1.18)

6% (uniform)10% (fracture)

T/Y = 1.10

1.33

1.18

Main Findings

• Walls designed using Grade 60 or Grade 100 reinforcement, 1.18, 6%, and 10% had similar strength

%).

Overview


Coupling Beams in Practice


Courtesy of: Pete Heeringa

Coupling Beam Demands

• Structural walls are a common lateral bracing system

• Often pierced to allow doors and windows

• Short “coupling beams” restore some of the stiffness and strength

Single Uncoupled Coupled

Coupling Beam Demands

• Wall deformations impose large deformation demands on coupling beams

• Beams are often short with span-to-depth ratios < 4

• High strength, stiffness and ductility requirements to maintain coupling

Park and Paulay, 1975


Why so much reinforcement?

L-Street apartment building in Anchorage, Alaska after 1964 earthquake

Courtesy of: EERI and IAEE, World Housing Encyclopedia, retrieved from http://db.world-housing.net/pdf_view/111/, February 2018.

Solution: High-Strength Steel

Grade 60

High-Strength

Steel

Coupling Beam Tests

In Practice Test Specimen

Coupling Beam Tests

Coupling Beam Tests

Span = 2.5*depth, Grade 100, 8 ksi concrete

ResultsGrade 80 Grade 100

10 10

-10 -5 0 5 10Chord Rotation, %

-250

-125

0

125

250

Shea

r, k

ip-20

-10

0

10

20

Shea

r St

ress

/ f

cm, p

si/p

si

Results

1010

Grade 80Grade 120

Findings/Recommendations

• Use of smaller amounts of HSS (such that is maintained) provides the expected strength and similar deformation capacity to conventional coupling beams

• Higher shear stresses may be permissible (and achievable) in HSS coupling beams

• Use of Grades 80 and 100 (and possibly 120) HSS reinforcement should be permitted in coupling beams

Overview


Shear Reinforcement – Mat Foundation

Objective

• Develop guidance for use of• Grade 80 steel as transverse reinforcement• Headed transverse reinforcement

• Headed transverse bars not engaging longitudinal reinforcement

Stirrups (hooked)

Headed (Engaged)

Headed (Not Engaged)

Scope and Test Setup

• 39 Specimens• Reinforcement:

Grades 60 and 80• Depths: 12 to 48 in.• Concrete compressive

strengths: 4 and 10 ksi

Observations

Stirrups (hooked)

Headed (Engaged)

Strength: Hooked vs. Headed

0.0

0.2

0.4

0.6

0.8

1.0

1.2

1.4

1.6

V T /V

n

=On average, headed shear

reinforcement led to the same strength as stirrups

SS HEHE HNEHNE

Strength: Grade 60 vs 80

0.0

0.2

0.4

0.6

0.8

1.0

1.2

1.4

1.6

V T /V

n

Grades 60 and 80 shear reinforcement

led to equivalent shear strength…

…when area of shear reinforcement decreased in proportion to

increase in grade

S HEHNE

= =

Crack Widths

36 in. depth, Grade 60 hooked stirrups, 4 ksi concrete

Crack Widths

Grade 60

Grade 80

Crack Widths

h = 48 in.

h < 20 in.

Findings/Recommendations

• Grade 60 and 80 stirrups provide equivalent strength and similar inclined crack widths (variables like beam depth had a much stronger influence than grade)

• Headed and hooked stirrups provide equivalent shear strength

• Both Grade 80 and headed stirrups should be permitted –with some limitations on headed stirrups (not discussed)

Overview

• High-Strength Steel (HSS) as Reinforcement• HSS in slender structural walls• HSS in coupling beams• HSS as shear reinforcement • Bond and development of HSS• Summary

Bond and Development of HSS

• Hooks:

• Heads:

• Straight:

y e c rdh b

c

fd

f

y edt b

c

fd

f

y t e sd b

c b tr

b

fd

f c Kd

y e cs odh b

c

fd

f

y e cs odt b

c

fd

f

y t e yd b

c b tr

b

fd

f c Kd

ACI 318-14 (fy ksi) ACI 318-19

Straight Bars in Tension

COV = 0.25

COV = 0.14

Straight Bars in Tension (Full Confinement)

0

10

20

30

40

50

60

70

80

0 5,000 10,000 15,000 20,000

ACI 318-14 Proposed

d/ d

b

fy = 60 ksi

0

10

20

30

40

50

60

70

80

0 5,000 10,000 15,000 20,000

d/ d

bfy = 100 ksi

Overview


Summary

• Use of Grades 80, 100, and 120 high-strength reinforcement in smaller amounts (such that is maintained) provides the expected strength and similar deformation capacity

Summary

• Grade 80 reinforcement is likely to be permitted by the ACI Building Code (318-19) for all applications

• ACI 318-19 may permit use of Grade 100 reinforcement in select applications (e.g., walls), with more applications likely permissible in future editions

• New laboratory facilities have allowed KU to be a key player in advancing these changes to ACI 318

Documents

High-Strength Steel in Structural Concrete · A615 and A706 A615 and A1035 A1035 Grade 100 High Strength. Key Parameters Yield strength Tensile strength Tensile-to-yield strength